1. Field of the Invention
The present invention generally relates to a method and apparatus for applying filaments about a cylindrical core and, in particular, to a method and apparatus for applying stacked bundles of filaments or ribbons into helical grooves in a slotted core or on the exterior surface of an elongated cylindrical strength member.
2. Description of the Prior Art
Devices to insert filaments, especially fiber optic ribbons, into grooves have been used in the past. These devices are broadly divided into two categories.
In the first category of devices, the fibers are loaded on a rotating cage while the slotted core does not rotate. The cage containing the fiber optic bobbins rotates around the slotted core following the slots. Such a system is shown in U.S. Pat. Nos. 4,587,801; 4,619,107 and 4,635,430. In the second category, the slotted core is made to rotate around its own axis while passing through the application point. The fibers in this case are carried by stationary bobbins and are inserted into the slots. Examples of this method are shown in U.S. Nos. 4,833,871 and 4,388,800.
For fiber optic filaments to be inserted into slotted cores, one usually assembles the fibers in bundles, stacks or packets and, in the last few years, fiber optic bundles have also been embedded in special plastic compounds, laid side-by-side to form a ribbon-like structure.
Fiber optic ribbons have been incorporated into cables in a variety of ways, one of which is to insert several stacked ribbons into slotted cores similar to the ones mentioned in the above prior art. Such ribbons have been known to be wound on cylindrical cores together with spacers that are inserted to create containment means to keep in place the packets of ribbons over the cylindrical support core. The application of ribbons and of packets of stacked ribbons requires specific care because ribbons, contrary to fiber optic bundles or fiber optics in a tube, can obviously be bent only in one direction and cannot be bent sideways. Therefore, the inserting devices that have been used for ribbon and ribbon packets are very complicated by the fact that it is very cumbersome to bring all the ribbon elements in a ringed configuration which will allow the insertion of the ribbon or ribbon packets around the circumference of the slotted or cylindrical core.
Furthermore, the application devices have also been complicated by the generally high number of guide pulleys that are required, making it very difficult to control and measure the actual tension with which the ribbons or the packets of ribbons are deposited in the slots or wound on the cylindrical core at the application point. One must bear in mind that when a packet or stack of ribbons are applied or wound onto a cylindrical surface, the speed of each ribbon in the packet increases slightly from the inner ribbon to the outer ribbon and, therefore, it is important that the ribbons in the packet are allowed to slide in relation to each other. In a circumferential insertion machine, this requires that each ribbon in the packet be brought in contact with the adjacent ribbon at the last possible moment, close to the application point. Therefore, the device becomes very cumbersome, particularly when many hundreds of ribbons might have to be brought together to converge at the closing point.
Most of the aforementioned apparatus for assembling or manufacturing fiber optic cables do not take into account that the optical properties of the fiber optic ribbons are very sensitive to any physical stress on the ribbons. However, because the ribbons are exposed to frictional forces as they are guided through the machine, the ribbons are subject to stresses and the tensions that are applied to the ribbons become and important consideration. A number of references have been discussed and have sought to address this problem. In U.S. Pat. No. 4,248,035, an apparatus is disclosed for assembling a fiber optic cable which seeks to avoid destructive tensile and compressive stresses in the fiber optic ribbons when the cable is bent and the grooves in which the fibers are inserted are helical in form. To minimize longitudinal tension in the fibers, air is directed along the surface of the cable in the feed direction. The tubes are sealed at the upstream end, the other tube extending beyond the inner tube by the downstream end. The air jet both frictionally urges fibers in the feed direction and creates a partial vacuum over the fibers to locally reduce contact pressure between the fibers and the central strength filament.
In U.S. Pat. No. 4,450,676, an apparatus is disclosed for stranding optical fiber cores while slackening them. A core delivery system is provided which has a passage for passing the optical fiber cores through it and a gas which flows from the back position of the passage towards the front functions to send out the optical fiber cores at the inlet side of the core storing spaces. The tensile forces on the optical fiber core at the inlet sides are decreased by a winding drum mechanism.
In U.S. Pat. No. 4,805,392, an optical fiber manufacturing apparatus is disclosed which seeks to prevent tensile stresses on the optical fibers. This is achieved by means of an inserting head, a photoelectric optical system between the supply bobbins and the inserting head to provide signals indicating the positions of the optical fibers advancing from the supply bobbins. A control device is connected to the photoelectric system and is responsive to the signals generated therefrom to control the speed at which the optical fibers are paid off from the supply bobbins.